Ceramics, glass and metal, for example, have been used as materials for electronic equipment and various glass compositions have been used as materials for bonding, sealing or coating them. The glass compositions have various forms such as bulk, powder, fiber and a thin film. There are also a material consisting of the glass composition only, and a composite material comprising a combination of the glass composition and other materials. Further, to give various functions depending on its applications, the glass composition may be dispersed in a vehicle together with other materials and an appropriate filler so that it can be used as a paste. The thus obtained paste may be used as a sealing member for magnetic heads, CRTs, liquid crystal display panels and plasma display panels.
Hereinafter, as a concrete example of applications of the sealing member comprising the glass composition, an explanation is given of the magnetic head and the plasma display panel.
First, a conventional magnetic head is explained.
A magnetic head is a device that performs recording and reproducing of magnetic information to and from a magnetic recording medium. The magnetic head comprises a pair of magnetic core halves, at least one of which being provided with a coil groove, and a magnetic gap member made of a non-magnetic material. The pair of magnetic core halves are abutted to each other via the magnetic gap member and bonded with a sealing member. As such a sealing member, a glass composition is used. The glass composition is an important constituent having an influence even on the properties of the magnetic head.
As a material for the magnetic core halves, ferrite has widely been used due to its excellent magnetic properties, wear resistance and machinability. A magnetic head manufactured with ferrite may be called a ferrite head.
In recent years, according to reduction in size and increase in capacity of the magnetic recording/reproducing device, a magnetic recording medium having high coercivity has come to be used. Aiming at a magnetic head for high density magnetic recording, which is highly capable of writing signals to such a medium, an extension of the ferrite head has been under development.
For example, a magnetic head called an MIG (metal in gap) head is formed by applying magnetic metal films having high saturated magnetic flux density to the surfaces of the magnetic core halves to be opposed to a magnetic gap, abutting the surfaces facing to the magnetic gap to each other via the magnetic gap member, and bonding them with a sealing member. As the magnetic metal film, for example, a thin film of a magnetic metal material such as Fe—Ta—N, Fe—Nb—N, Fe—Nb—Si—B—N, Fe—Ta—C, Co—Ta—Zr—Nb or Co—Nb—Zr—N is used.
There is also a magnetic head comprising magnetic core halves, each of which comprising a magnetic metal film sandwiched between a pair of nonmagnetic substrates. The magnetic core halves are arranged such that the magnetic metal films are abutted at their end faces and bonded with a sealing member via a magnetic gap member. Such a magnetic head is called a stacked head.
In the process of manufacturing the magnetic head, the sealing member is used by softening, liquidizing, cooling and solidifying the glass composition by an appropriate heat treatment. Therefore, to prevent deterioration of the magnetic substance composing the magnetic head by heat, a glass composition usable at a temperature as low as possible has been demanded. As such a glass composition that meets the demand, there is a so-called low softening point glass.
In general, the glass composition is apt to have a higher thermal expansion coefficient when the softening point is lower. However, in order to avoid breaking or cracking due to warpage after cooling, it is necessary to control the thermal expansion coefficient of the sealing member not to increase. Further, to exhibit optimum magnetic recording properties, it is necessary to control the warpage of the magnetic substance caused by a difference in thermal expansion coefficient from the sealing member. Therefore, there has been a demand for a sealing member having a thermal expansion coefficient suitable for the specifications of various magnetic heads.
The sealing members of the magnetic heads are required to have temperature properties and thermal expansion coefficients suitable for their applications, respectively. The sealing member used for the magnetic heads such as the ferrite head, MIG head and stacked head is required to have a working temperature of 450 to 650° C. and a thermal expansion coefficient of 70×10−7 to 130×10−7/° C. For example, as to the sealing member for the MIG head, it is preferred that the working temperature is about 500° C. and the thermal expansion coefficient is 75×10−7 to 100×10−7/° C.
The working temperature mentioned herein is a temperature at which the viscosity of the glass composition becomes 103 Pa·s. Further, the softening point is a temperature obtained by measurement based on a JIS test method R3103-1, which is determined as a temperature at which the viscosity of the glass composition becomes 106.6 Pa·s. With the exception of the case where the viscosity behavior of the glass composition is unique, a glass composition having a low softening point is apt to have a low working temperature.
Next, a common plasma display panel (hereinafter may be referred to as PDP) is explained.
FIG. 1 is an example of a partial schematic oblique view illustrating a major structure of a common PDP.
In FIG. 1, the PDP comprises a front plate 1 and a rear plate 8 bonded to each other.
The front plate 1 includes a front glass substrate 2, display electrodes 5 formed on a single surface thereof, each of which comprising a transparent conductive film 3 and a bus electrode 4, a dielectric layer 6 made of dielectric glass covering the display electrodes 5 and a dielectric layer protecting layer 7 made of magnesium oxide.
The rear plate 8 includes a rear glass substrate 9, address electrodes 10 formed on a single surface thereof, a dielectric layer 11 covering the address electrodes 10, barrier ribs 12 arranged at regular intervals on the top surface of the dielectric layer 11 to form discharge spaces 14 and fluorescent layers formed inside the discharge spaces 14 for color display. The fluorescent layers include a red fluorescent layer 13(R), a green fluorescent layer 13(G) and a blue fluorescent layer 13(B) arranged sequentially, which are excited by ultraviolet light having a short wavelength of 147 nm, for example, to emit light.
The front plate 1 and the rear plate 8 are arranged such that the lengthwise directions of the display electrodes 5 and the address electrodes 10 are orthogonal to each other, and then bonded with a sealing member comprising a glass composition. FIG. 2 shows the arrangement of the display electrodes and the address electrodes, with an illustration of a sealing part.
Referring to FIG. 2, a sealing part 15 is the periphery of an overlapped region between the stacked front plate 1 and the rear plate 8. A paste comprising the sealing member is applied to the sealing part 15 and the front plate 1 and the rear plate 8 are bonded, thereby the inside thereof is sealed. Then, the display electrodes 5 and the address electrodes 10 are connected to external driving circuits (not shown), respectively.
FIG. 3 is an example of a cross section cut along the address electrode of the PDP shown in FIG. 1 illustrating the vicinity of the sealing part.
At least one of the front plate 1 and the rear plate 8 is provided with an air hole 17 for exhausting air from the end of a glass tube 18 communicating with the air hole 17 is fixed to the periphery of the opening end of the air hole 17 with a sealing member 16. Then, while heating to a predetermined temperature, air is exhausted from the inside of the PDP. After the exhaustion is completed, the discharge gas, is introduced up to a predetermined pressure. Then, finally, a basal portion of the glass tube 18 is heat-sealed.
In the thus manufactured PDP, discharge is performed: for a predetermined time to stabilize light-emitting properties and discharge properties as required.
In the process of manufacturing the PDP, the heat treatment in the sealing step using the sealing member 16 needs to be conducted at low temperature so as not to cause any trouble to the other members. That is, since the PDP contains glass materials in the glass substrates, electrodes, dielectric layers and barrier ribs, it is necessary to prevent these glass materials from being softened or deformed by heating in the sealing step. Further, it is also necessary to prevent deterioration of the dielectric layer protecting layer 7 and the fluorescent layers 13(R), 13(G) and 13(B). In general, it is desirable to carry out the sealing step at a treatment temperature of 500° C. or lower.
Thus, in the step of sealing the magnetic head or the PDP, it is preferable to use the sealing member at low temperature. Therefore, low softening point glass is used for the glass composition.
For example, examples of the low softening point glass used for the sealing member for the magnetic head include lead glasses such as SiO2—B2O3—PbO glass (Japanese Laid-Open Patent Publication No. HEI 8-180310) and B2O3—PbO—ZnO glass. Further, as the sealing glass for the PDP, lead glass containing PbO as a main component is used. That is, it is essential to contain lead to achieve the low softening point.
Although lead is heavily used as a component of the low softening point glass, it has been pointed out that lead has toxicity to human beings and hazardous properties to environment. Further, working environment in the manufacture of the magnetic head or the PDP and influence on the environment that accompanies disposal of the products have been brought into question. Therefore, there has been a demand for a sealing member using a lead free glass composition.
As the lead free low-melting glass, phosphate glass has been developed. However, in actual use, it is not sufficiently reliable, particularly with respect to water resistance. Moreover, a sealing member comprising a glass composition having low water resistance, for example, is apt to absorb moisture in the atmosphere in the manufacturing process of the PDP and the moisture may possibly remain in the PDP to cause harmful effect on display performance. In order to prevent this, the glass composition contained in the sealing member is required to be excellent in water resistance.
Further, there has also been studied a glass composition comprising lead free, low-melting point bismuth glass (Japanese Laid-Open Patent Publication No. HEI 10-139478).